1. Field of the Invention
The invention relates to a method of transporting and installing an offshore structure, such as an offshore production platform, in particular for oil or gas.
2. Discussion of the Prior Art
The offshore oil industry is highly competitive and therefore the cost of constructing, transporting and installing offshore structures is extremely important. Moreover, the cost of transporting and installing a platform may be even higher than its construction costs.
One known method of installing offshore platforms involves lifting the platform sections by a crane vessel and assembling them in situ. However this is expensive since it involves the long term use of installation vessels, which are expensive to hire. Another method involves building buoyancy tanks into a structure to allow it to be floated to its desired position and then ballasting the structure onto the water bed However this increases the cost of constructing the platform.
The present invention seeks to provide a method of transportation and installation which is both simple and which is less expensive than known methods.
From a first aspect, therefore, the invention provides a method of transporting and installing an offshore structure comprising transporting the offshore structure to the installation site by barge with at least a portion of the structure being supported under the barge, and lowering the offshore structure to the water bed.
Thus the invention contemplates supporting at least a part of a structure below a barge, conveying the structure to its installation site and then lowering the structure into position.
This method has several advantages over the known methods described above. Firstly, it allows a standard barge to be used for transportation purposes, which is extremely cost effective. A fully assembled structure may be transported to the installation site and no assembly of the structure need take place at the installation site. Furthermore, the structure may be simply lowered into position from the barge, avoiding the need for expensive machinery for lifting the is structure from the barge. Thus the method of the invention offers significant advantages over the known methods.
The structure may be supported to the barge in a number of ways. In the preferred method of attachment, the barge is floated into position over the structure and the structure then attached to the barge. Thus preferably, the structure is constructed in a dry dock which is then flooded and the barge is floated over the appropriate part of the structure.
Although it would be possible to suspend the structure below the barge, for stability reasons, the structure is preferably made fast with the barge. To this end, the barge is preferably lowered over the structure into contact with an upper surface thereof and the structure then made fast with the barge. It may be possible, for example, to lower the barge over the offshore structure by at least partially emptying the dry dock so that the barge sins with the sinking water level. However most simply, the barge is ballasted down to the structure. The barge and structure are then attached to each other.
The structure and barge may be attached to each other by any means, for example by welding. Preferably however the offshore structure is attached and tensioned to the barge by standard cables using jacking means. The tensioning of the barge to the structure allows relative movement between them during transportation to be minimised. In addition, as will be seen, the jacking means can also be used to lower the structure to the water bed at the installation site.
In a preferred embodiment the barge and offshore structure are towed to the installation site by tugs. It will however be appreciated that any means of conveying them to the installation site might be employed. For example, they could be self propelled.
It will be understood from the above that the method of the invention could be used to install many forms of offshore structure. However, it is expected to be most useful for the installation of offshore production platforms.
A preferred structure for installation by the invention comprises a base, preferably of concrete, which when installed rests on the water bed and a tower, preferably a steel tower extending upwardly from the base and mounting suitable topsides. In such a structure the base or a portion thereof is supported beneath the barge, and the tower extends to one side or end of the barge. Thus the base may extend either sideways or lengthwise underneath the barge.
To facilitate such an arrangement, the tower of the platform is preferably offset from the centre of the platform base Since standard platforms known in the art are conventionally constructed with their tower at the centre of the platform base, this arrangement is believed to be novel and inventive in its own right, so from a further aspect the invention also provides an offshore platform comprising a base and a tower wherein the tower is offset from the centre of the base.
In order to counter the pitching or rolling moment generated by the tower offset during transportation, and thus improve the stability of the barge and platform while being conveyed to the installation site, the base is preferably provided with a suitably positioned counterweight. The counterweight may conveniently be formed as an upstand formed over part of the platform base. This may be sized and positioned such that when the platform is supported to the barge, it extends up the side of the barge opposite the side over which the tower extends.
In an alternative arrangement, when the base extends lengthwise under the barge however, the length of the base itself will act, to some extent as a counterweight, obviating the need for an upstanding counterweight.
To assist in installation, a positioning means may be provided on the water bed prior to the installation. The positioning means could be installed by any method, for example, lifting it into place or piling offshore. However it is preferred that it is also installed by the method of the invention.
The positioning means preferably comprises a large mass, for example of concrete, dimensioned to resist the forces generated during installation of the platform and having guide means for engagement with cooperating guide means on the platform.
As in the present invention, the base is held underneath the barge during transportation, the buoyancy of the structure is provided by the barge. Therefore, the thickness of the concrete base may be minimised to that required in the installed condition. However, traditionally the steel tower of an offshore structure is attached to the concrete base by being cast into the concrete during manufacture of the base. This method of attachment, however, requires the concrete base to have a thickness of at least 2 to 3 times the diameter of the steel tower Such a method is not, therefore, suitable for towers with relatively thin bases.
Thus in a preferred structure of the invention, the tower is attached to the base by bolting, most preferably by bolts extending upwardly from the base.
This method is believed to be novel and inventive in its own right. Thus, from a still further aspect, the present invention provides an offshore structure comprising a base and a tower extending upwardly therefrom, wherein the tower is attached to the base by bolts, most preferably by bolts extending upwardly from the base.
In this method, the bolts may be securely fastened in the base, for example by being pre-cast into the base or by being grouted into bores provided in the base, and the tower then assembled over the upstanding bolts and fastened into position. To maximise the strength of the mounting, the mounting bolts preferably extend substantially through the base.
To prevent corrosion, the portions of the bolts extending above the base may be coated with a suitable resistant coating.
The base of the tower is preferably formed with a fastening flange extending radially outwardly therefrom, for receiving the bolts. Most preferably circumferential stiffening ribs are provided between the flange and the tower to provide rigidity in that region, and bolt receiving holes are formed in the flange in the pockets defined between adjacent stiffening ribs.
In assembling the tower, the tower base flange may be assembled over the upstanding bolts, with a layer of grout positioned between the tower base flange and the upper surface of the concrete base to provide a satisfactory seating for the flange on the concrete base. An anchor block may then be positioned over the ends of the bolts to rest on the flange. An epoxy seating or the like may be provided for the block so as to prevent ingress of water to the bolt, thereby preventing corrosion.
After nuts have been tightened onto the bolts to securely attach the tower to the base, an enclosure may be defined around the nut and exposed bolt, which is filled with epoxy or some other sealant, again to prevent corrosion.
It will be appreciated that the tower need not be completely prefabricated before it is attached to the base. Only a tower base part need be constructed and attached and further tower sections then attached to the tower base part, for example by welding.
A preferred embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:
FIG. 1 is a schematic representation of a first stage of an installation sequence for an offshore platform;
FIG. 2 is a schematic representation of a second stage of the installation sequence for an offshore platform;
FIG. 3 is a schematic representation of a third stage of the installation sequence for an offshore platform;
FIG. 4 is a schematic representation of a fourth stage of the installation sequence for an offshore platform;
FIG. 5 is a schematic representation of a fifth stage of the installation sequence for an offshore platform;
FIG. 6 is a schematic representation of a sixth stage of the installation sequence for an offshore platform.
FIG. 7 is a plan view of an offshore platform of the invention;
FIG. 8 is a sectional view showing the attachment method of the present invention.